Modified colloidal silica and method for producing the same, and polishing agent using the same

ABSTRACT

To provide modified colloidal silica capable of improving the stability of the polishing speed with time when used as abrasive grains in a polishing composition for polishing a polishing object that contains a material to which charged modified colloidal silica easily adheres, such as a SiN wafer, and to provide a method for producing the modified colloidal silica. 
     Modified colloidal silica, being obtained by modifying raw colloidal silica, wherein
         the raw colloidal silica has a number distribution ratio of 10% or less of microparticles having a particle size of 40% or less relative to a volume average particle size based on Heywood diameter (equivalent circle diameter) as determined by image analysis using a scanning electron microscope.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/544,389, filed Jul. 18, 2017, which is a 371 of PCT/JP2016/051442,filed Jan. 19, 2016, which claims the benefit of Japanese PatentApplication No. 2015-008049, filed Jan. 19, 2015, the contents of whichare hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to modified colloidal silica and a methodfor producing the same, and a polishing agent using the same.

BACKGROUND ART

In a semiconductor device production process, as the performance of asemiconductor device is improved, techniques for producing the wiringwith higher density and higher integration are required. In a productionprocess of such a semiconductor device, chemical mechanical polishing(CMP) is an essential process. As the miniaturization of thesemiconductor circuit progresses, it is demanded to realize the highflatness required for the unevenness of a pattern wafer and also torealize the high smoothness of nano order by CMP. In order to realizethe high smoothness by CMP, it is preferred that the convex portion ofthe pattern wafer is polished at a high polishing speed but the concaveportion is not polished so much.

Herein, for example, in a case of using a pattern wafer made of asilicon nitride film (SiN film), since the silicon nitride film usuallyhas unevenness, when polishing such a material, not only the convexportions but also the concave portions are scraped together, and theunevenness are hardly sufficiently eliminated.

In addition, the semiconductor wafer is constituted of dissimilarmaterials including polycrystalline silicon forming a circuit, siliconoxide being an insulating material, and silicon nitride for protecting asilicon dioxide surface that is not part of the trench or the via fromthe damages during etching. Therefore, a phenomenon such as dishing, inwhich a material that is relatively soft and easily reacts with apolishing agent, such as polycrystalline silicon, and silicon oxide isscraped excessively as compared with the silicon nitride or the likesurrounding the material, is caused, and unevenness is left.

In view of the above, in a polishing process of a pattern wafer made ofa hard and chemically stable material such as silicon nitride, it isrequired to sufficiently eliminate the unevenness.

As a technique to respond to the requirement, for example, in JP2012-040671 A, for the purpose of providing a polishing compositioncapable of polishing at a high speed a polishing object that is poor inchemical reactivity, such as silicon nitride, a technique in whichcolloidal silica (sulfonic acid group (anionic) modified colloidalsilica) obtained by immobilizing an organic acid is contained in acomposition as abrasive grains, and the pH of the composition is 6 orless has been disclosed.

Herein, in general, there is a problem that silica sol such as colloidalsilica is unstable because silica particles aggregate with each otherunder an acidic condition. As a technique to solve such a stabilityproblem, in JP 2010-269985 A, sulfonic acid-modified aqueous anionic solhaving a zeta potential of −15 mV or less at an acidic of pH 2 or morehas been disclosed. In addition, in JP 2010-269985 A, as a method forproducing the anionic sol described above, a technique in which a silanecoupling agent having a functional group chemically convertible to asulfonic acid group (for example, a mercapto group) is added tocolloidal silica, and then the functional group is converted to asulfonic acid group has been disclosed. Herein, in Examples of JP2010-269985 A, silica sol containing water and methanol as a dispersingmedium is heated and concentrated under alkaline and normal pressureconditions, and then into the silica sol, a mercapto group-containingsilane coupling agent (3-mercaptopropyl trimethoxysilane) is added, andthe resultant mixture is refluxed at a boiling point and heat aged.Next, methanol and ammonia are replaced with water, and the resultantmixture is cooled down to room temperature when the pH becomes 8 orless, and into the cooled mixture, hydrogen peroxide water is added, themercapto group is converted to a sulfonic acid group by heating themixture, as a result, anionic silica sol of which the surface has beenmodified with a sulfonic acid group is obtained.

In addition, in JP 2013-41992 A, there is a disclosure about theproduction of similar sulfonic acid-modified aqueous anionic silica sol,referring to the above-described JP 2010-269985A, and J. Ind. Eng.Chem., Vol. 12, No. 6 (2006) 911-917. Herein, in Examples of JP2013-41992 A, into an aqueous solution of the mercapto group-containingsilane coupling agent similar to that as described above (under anacidic condition with acetic acid), silica sol containing water as adispersing medium is added, the resultant mixture is stirred at roomtemperature for one hour, and then into the mixture hydrogen peroxidewater is added, and the resultant mixture is left to stand at roomtemperature for 48 hours, as a result, sulfonic acid-modified aqueousanionic silica sol is obtained.

SUMMARY OF INVENTION

In performing the technique described in JP 2012-040671 A, the presentinventors tried to use the modified colloidal silica produced by themethod described in JP 2010-269985 A and JP 2013-41992 A. As a result,it was found that there is a problem that when a SiN wafer is polishedby using a polishing composition containing anionic modified colloidalsilica that has been produced by using the techniques described in theseconventional arts, the ratio of the polishing rates of SiN to tetraethylorthosilicate (TEOS) or polycrystalline silicon (poly-Si) fluctuatesover time.

Accordingly, an object of the present invention is to provide modifiedcolloidal silica capable of improving the stability of the polishingspeed with time when used as abrasive grains in a polishing compositionfor polishing a polishing object that contains a material to whichcharged modified colloidal silica easily adheres, such as a SiN wafer,and to provide a method for producing the modified colloidal silica.

To solve the problem described above, the present inventors carried outintensive studies. As a result, it was found that by performing amodification treatment using colloidal silica in which the amount of themicroparticles has been reduced as raw colloidal silica, modifiedcolloidal silica is obtained, and thus the modified colloidal silicawhich can solve the above-described problems can be obtained. Based onthe above findings, the present inventors thus have completed thepresent invention.

That is, according to an aspect, there is provided modified colloidalsilica, being obtained by modifying raw colloidal silica, wherein

the raw colloidal silica has a number distribution ratio of 10% or lessof microparticles having a particle size of 40% or less relative to avolume average particle size based on Heywood diameter (equivalentcircle diameter) as determined by image analysis using a scanningelectron microscope.

Furthermore, according to another aspect, there is provided a method forproducing modified colloidal silica, including the steps of:

distilling off an organic solvent coexisting with colloidal silica incolloidal silica having an organic solvent concentration of 1% by massor more under a condition of pH 7 or more so that a residual organicsolvent concentration is less than 1% by mass to obtain raw colloidalsilica; and

modifying the raw colloidal silica to obtain modified colloidal silica.

According to the modified colloidal silica and method for producing thesame according to the present invention, in a polishing composition forpolishing a polishing object that contains a material to which chargedmodified colloidal silica easily adheres, such as a SiN wafer, when themodified colloidal silica is used as abrasive grains, the stability ofthe polishing speed with time can be improved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a picture of sulfonic acid-modified colloidal silica obtainedin Example 1, observed with a scanning electron microscope (SEM)(magnification: 100000 times).

FIG. 2 is a picture of sulfonic acid-modified colloidal silica obtainedin Example 1, observed with a transmission electron microscope (TEM)(magnification: 400000 times).

FIG. 3 is a picture of sulfonic acid-modified colloidal silica obtainedin Comparative Example, 1 observed with a scanning electron microscope(SEM) (magnification: 100000 times).

FIG. 4 is a picture of sulfonic acid-modified colloidal silica obtainedin Comparative Example 1, observed with a transmission electronmicroscope (TEM) (magnification: 400000 times).

FIG. 5 is a picture of sulfonic acid-modified colloidal silica obtainedin Comparative Example 2, observed with a scanning electron microscope(SEM) (magnification: 100000 times).

FIG. 6 is a picture of sulfonic acid-modified colloidal silica obtainedin Comparative Example 2, observed with a transmission electronmicroscope (TEM) (magnification: 400000 times).

DESCRIPTION OF EMBODIMENTS

Hereinafter, the embodiment for carrying out the present invention willbe described in detail.

An embodiment of the present invention is modified colloidal silicaobtained by modifying raw colloidal silica having a number distributionratio of 10% or less of microparticles having a particle size of 40% orless relative to a volume average particle size based on Heywooddiameter (equivalent circle diameter) as determined by image analysisusing a scanning electron microscope. In addition, in the presentspecification, the expression “obtained by modifying raw colloidalsilica” means a state in which a modifying group is bonded to a surfaceof particles of raw colloidal silica. For example, in a case of theexpression “obtained by anionically modifying raw colloidal silica”,means a state in which an anionic group (for example, a sulfonic acidgroup) is bonded to a surface of particles of raw colloidal silica, andin a case of the expression “obtained by cationically modifying rawcolloidal silica”, means a state in which a cationic group (for example,an amino group or a quaternary cationic group) is bonded to a surface ofparticles of raw colloidal silica.

[Raw Colloidal Silica]

The raw colloidal silica can be, for example, colloidal silica producedby a sol-gel method. The raw colloidal silica produced by a sol-gelmethod is preferred because the content of diffusible metal impuritiesand the content of corrosive ions such as chloride ions in asemiconductor are small. The production of the raw colloidal silica by asol-gel method can be performed by using a conventionally knowntechnique, specifically, by using a hydrolyzable silicon compound (forexample, an alkoxysilane or a derivative thereof) as a raw material,hydrolysis and condensation reaction is performed, as a result of whichthe raw colloidal silica can be obtained. The silicon compound may beused singly alone, or may also be used in combination of two or morekinds thereof. Further, the raw colloidal silica may also be the oneproduced by a method other than the sol-gel method.

In an embodiment, the silicon compound is preferably an alkoxysilanerepresented by the following general formula (1) or a derivativethereof.Si(OR)₄  (1)

In the general formula (1), R is an alkyl group, preferably a loweralkyl group having 1 to 8 carbon atoms, and more preferably a loweralkyl group having 1 to 4 carbon atoms. Herein, examples of the Rinclude methyl group, ethyl group, propyl group, isopropyl group, butylgroup, pentyl group, and hexyl group, and tetramethoxysilane in which Ris methyl group, tetraethoxysilane in which R is ethyl group, andtetraisopropoxysilane in which R is isopropyl group are preferred.Further, as the derivative of the alkoxysilane, a low condensateobtained by partially hydrolyzing the alkoxysilane is exemplified. Inthe present invention, it is preferred to use tetramethoxysilane fromthe point that it is easy to control the hydrolysis rate, the point thatfine silica particles of single nm can be easily obtained, and the pointthat unreacted residues are less.

The silicon compound is hydrolyzed and condensed in a reaction solvent,and becomes colloidal silica. As the reaction solvent, water or anorganic solvent containing water can be used. Examples of the organicsolvent include a hydrophilic organic solvent including alcohols such asmethanol, ethanol, isopropanol, n-butanol, t-butanol, pentanol, ethyleneglycol, propylene glycol, and 1,4-butanediol; ketones such as acetone,and methyl ethyl ketone; and the like. Among these organic solvents, itis particularly preferred to use alcohols such as methanol, ethanol, andisopropanol, and from the viewpoint of the post-processing of thereaction solvent, and the like, it is more preferred to use alcohols(for example, methanol to tetramethoxysilane) having the same alkylgroup as the alkyl group (R) of the raw silicon compound. These organicsolvents may be used singly alone, or may also be used in combination oftwo or more kinds thereof. The use amount of the organic solvent is notparticularly limited, but is preferably around 5 to 50 mol per 1 mol ofthe silicon compound. When the use amount is 5 mol or more, thesufficient compatibility with the silicon compound is ensured, and whenthe use amount is 50 mol or less, the decrease in the productionefficiency is suppressed. The amount of the water to be added into theorganic solvent is not particularly limited, as long as the amountrequired for the hydrolysis of the silicon compound is contained, andaround 2 to 15 mol per 1 mol of the silicon compound is preferred. Inaddition, the amount of the water to be mixed in the organic solventlargely affects the particle size of the colloidal silica to be formed.By increasing the addition amount of water, the particle size of thecolloidal silica can be increased. By decreasing the addition amount ofwater, the particle size of the colloidal silica can be reduced.Accordingly, by changing the mixing ratio of the water and the organicsolvent, the particle size of the colloidal silica to be produced can bearbitrarily adjusted.

It is preferred to adjust the reaction solvent to alkaline by adding abasic catalyst to the reaction solvent of the hydrolysis condensationreaction of the silicon compound to obtain colloidal silica (Stobermethod). Accordingly, the reaction solvent is adjusted to preferably pH8 to 11, and more preferably to pH 8.5 to 10.5, and the colloidal silicacan be rapidly formed. As the basic catalyst, organic amine and ammoniaare preferred from the viewpoint of preventing the contamination ofimpurities, in particular, ethylenediamine, diethylenetriamine,triethylenetetramine, ammonia, urea, ethanol amine, tetramethylammoniumhydroxide, and the like can be preferably mentioned.

In order to hydrolyze and condense the silicon compound in the reactionsolvent, the silicon compound that is a raw material is added to anorganic solvent, and the resultant mixture is stirred at a temperaturecondition of 0 to 100° C., and preferably 0 to 50° C. By hydrolyzing andcondensing the silicon compound while stirring the silicon compound inan organic solvent containing water, colloidal silica having a uniformparticle size can be obtained.

As described above, the modified colloidal silica according to thepresent embodiment is also obtained in the similar manner as thetechnique described in JP 2010-269985 A, by subjecting the raw colloidalsilica produced by a sol-gel method as in the above to a modificationtreatment. However, according to the present embodiment, in the point ofusing as the raw colloidal silica the one in which the amount of themicroparticles has been reduced, an improvement has been made to thetechnique described in JP 2010-269985 A. Specifically, the modifiedcolloidal silica according to the present embodiment is characterized bybeing obtained by modifying the raw colloidal silica having a numberdistribution ratio of 10% or less of the microparticles having aparticle size of 40% or less relative to a volume average particle size(hereinafter, also simply referred to as “microparticles”) based onHeywood diameter (equivalent circle diameter) as determined by imageanalysis using a scanning electron microscope. The number distributionratio is preferably 5% or less, more preferably 2% or less, furthermorepreferably 1% or less, still more preferably 0.5% or less, particularlypreferably 0.3% or less, and most preferably 0.2% or less. From theviewpoint of obtaining action effects of the present invention, thesmaller the number distribution ratio of the microparticles is, the morepreferred it is, therefore, the lower limit value of the numberdistribution ratio is not particularly limited, but is, for example,0.001% or more. In addition, the method for measuring the numberdistribution ratio is performed in accordance with the description inExamples described later.

There is no particular limitation on the specific technique foradjusting the number distribution ratio of the microparticles containedin the raw colloidal silica to 10% or less, and conventionally knownknowledge can be appropriately referred. As an example of such atechnique, in a case where the concentration of the organic solvent inthe colloidal silica produced by the above-described hydrolysis andcondensation reaction (sol-gel method) is 1% by mass or more, a methodfor removing the organic solvent coexisting with the colloidal silicacan be mentioned so that the residual organic solvent concentration inthe colloidal silica is less than 1% by mass. Herein, the expression“whether or not the residual organic solvent concentration in thecolloidal silica is less than 1% by mass” is synonymous with theexpression “whether or not the organic solvent is detected in thecolloidal silica” in the method for measuring the organic solventconcentration (methanol concentration in Examples) using the gaschromatography that is described in Examples described later. That is,the above-described expression “so that the residual organic solventconcentration in the colloidal silica is less than 1% by mass” can alsobe paraphrased as the expression “so that the organic solvent in thecolloidal silica measured by the measurement method described in theExamples using the gas chromatography, is lower than the detectionlimit”.

As described above, by decreasing the concentration of the organicsolvent contained in the colloidal silica, the amount of themicroparticles contained in the raw colloidal silica can be decreased.At this time, as the amount of the organic solvent contained in thecolloidal silica is decreased, the amount of the microparticlescontained in the raw colloidal silica can be decreased. In addition, theorganic solvent concentration in the colloidal silica obtained by theabove-described Stober method usually exceeds 1% by mass. Therefore,according to another embodiment of the present invention, a method forproducing modified colloidal silica, in which through a step of removingthe organic solvent coexisting with the colloidal silica in thecolloidal silica having the residual organic solvent concentration of 1%by mass or more such as the colloidal silica obtained by a Stobermethod, so that the residual organic solvent concentration, is 1% bymass or less, raw colloidal silica is obtained, and then modifiedcolloidal silica is obtained by modifying the raw colloidal silica, isalso provided.

As the technique for removing the organic solvent coexisting with thecolloidal silica, a method in which a dispersion (silica sol) ofcolloidal silica is heated and the organic solvent is distilled off canbe mentioned. At this time, by replacing the organic solvent to beremoved with water, the liquid amount of the colloidal silica dispersioncan be maintained. In addition, the pH of the colloidal silicadispersion at the time of distilling off the organic solvent ispreferably pH 7 or more. As a result, there is an advantage thattogether with the distillation of the organic solvent, themicroparticles can also be incorporated on surfaces of the mainparticles of colloidal silica by Ostwald ripening, and the amount of themicroparticles can further be decreased.

In the above description, as the technique for adjusting the numberdistribution ratio of the microparticles contained in the raw colloidalsilica to 10% or less, a method for removing the organic solventcoexisting with the colloidal silica has been explained in detail as anexample, but by a technique different from this, the number distributionratio of the microparticles contained in the raw colloidal silica may beset to 10% or less. As such a technique, for example, a technique ofusing oligomers as a raw material, a technique of optimizing thecomposition at the time of synthesis, a technique of performing a hightemperature and pressure treatment after synthesis, a technique ofperforming centrifugation after synthesis, and the like can bementioned, and a technique other than these techniques may also be usedof course.

[Modification Treatment]

As described above, the modified colloidal silica according to thepresent embodiment can be obtained by subjecting the raw colloidalsilica produced by a sol-gel method to a modification treatment, but thespecific embodiment of the modification treatment is not particularlylimited, and among the conventionally known modification treatments ofcolloidal silica, a treatment capable of subjecting the colloidal silicato anionic modification or cationic modification can be appropriatelyused.

(Anionically-Modified Colloidal Silica)

In the following, as one preferred embodiment of the modificationtreatment, an example of a technique for obtaining anionically-modifiedmodified colloidal silica by modifying the raw colloidal silica with asulfonic acid group will be described. In this technique, themodification step includes a first reaction step of heating the rawcolloidal silica in the presence of a silane coupling agent having afunctional group chemically convertible to a sulfonic acid group toobtain a reactant by, and a second reaction step of treating thereactant to convert the functional group to a sulfonic acid group.

(First Reaction Step)

In the first reaction step, the raw colloidal silica is heated in thepresence of a silane coupling agent having a functional group chemicallyconvertible to a sulfonic acid group. As a result, a reactant (in whicha silane coupling agent having a functional group chemically convertibleto a sulfonic acid group is bonded to surfaces of silica particles) canbe obtained.

Herein, as needed, before the first reaction step, various treatmentsteps may be applied to the raw colloidal silica obtained in the aboveprocedure. As such a treatment step, for example, a step of lowering theviscosity of the raw colloidal silica can be mentioned. As the step oflowering the viscosity of the raw colloidal silica, for example, a stepof adding an alkaline solution (aqueous solution of various bases, suchas ammonia water) or an organic solvent to the raw colloidal silica canbe mentioned. The amount of the alkaline solution or organic solvent tobe added at this time is not particularly limited, and may beappropriately adjusted in consideration of the viscosity of the rawcolloidal silica to be obtained after the addition. As described above,by performing the step of lowering the viscosity of the raw colloidalsilica, there is an advantage that the initial dispersibility of acoupling agent to colloidal silica is improved and the aggregation ofsilica particles can be suppressed.

In the first reaction step, as described above, the raw colloidal silicahaving a small content of microparticles is heated in the presence of asilane coupling agent having a functional group chemically convertibleto a sulfonic acid group. As a result, a reactant can be obtained. Asdescribed above, the reason why the silane coupling agent having afunctional group different from a sulfonic acid group is reacted withthe raw colloidal silica, and then the functional group is converted toa sulfonic acid group (the second reaction step described later) isbecause in general, it is difficult to stably obtain a silane couplingagent in a form of being replaced with sulfonic acid groups.

As the silane coupling agent having a functional group chemicallyconvertible to a sulfonic acid group, for example, 1) a silane couplingagent having a sulfonic acid ester group convertible to a sulfonic acidgroup by hydrolysis, and 2) a silane coupling agent having a mercaptogroup and/or a sulfide group convertible to a sulfonic acid group byoxidation can be mentioned. In addition, since the sulfonic acidmodification on the colloidal silica surface is performed in a solution,in order to increase the modification efficiency, it is preferred to usethe latter coupling agent having a mercapto group and/or a sulfidegroup.

Examples of the silane coupling agent having a mercapto group include3-mercaptopropyl trimethoxysilane, 2-mercaptopropyl triethoxysilane,2-mercaptoethyl trimethoxysilane, and 2-mercaptoethyl triethoxysilane.Further, as the coupling agent having a sulfide group, for example,bis(3-triethoxysilylpropyl)disulfide, or the like can be mentioned.

Further, the silane coupling agent is hydrolyzed in advance with anacid, and then the condensation reaction to the raw colloidal silica mayalso be performed.

As described above, as the technique for adjusting the numberdistribution ratio of the microparticles contained in the raw colloidalsilica to 10% or less, in a case of employing a method for removing theorganic solvent coexisting with colloidal silica, the raw colloidalsilica substantially does not contain the organic solvent, and thedispersing medium of the raw colloidal silica is substantially composedof water. On the other hand, since the silane coupling agent is hardlydissolved in water, for the purpose of dissolving the silane couplingagent, a certain amount or more of an organic solvent (hydrophilicsolvent) is preferably used. As such an organic solvent (hydrophilicsolvent), for example, the above-described organic solvents such asmethanol, ethanol, and isopropanol can be exemplified. Among them, it ispreferred to use the same kind of alcohol as the alcohol produced by theabove-described hydrolysis of silicon compound. This is because therecovery and reuse of the solvent can be facilitated by using the samekind of alcohol as the alcohol produced by the hydrolysis of siliconcompound. Further, such an organic solvent (hydrophilic solvent) may beadded to the raw colloidal silica, or the silane coupling agent is mixedin advance with the organic solvent (hydrophilic solvent) to obtain amixture, and the mixture may be added to the raw colloidal silica. Thelatter method is more preferred. In addition, in JP 2010-269985 A, thereis a description of “Considering the solubility of the coupling agent,it is preferred that the colloidal silica contains a hydrophilic organicsolvent. In this regard, in a case where colloidal silica is obtained bya Stober method in which an alkoxysilane is hydrolyzed and condensed inan alcohol-water solvent with a basic catalyst, alcohol is contained inthe reaction mixture, therefore, it is not required to further add ahydrophilic organic solvent.” However, in the preferred embodiment ofthe present invention, a seemingly roundabout configuration in which theamount of the organic solvent contained in the raw colloidal silicaobtained by a Stober method is once decreased to lower than thedetection limit, and then the silane coupling agent is added isemployed. According to the preferred embodiment of the presentinvention, even if the roundabout configuration as described above isemployed, it has been found that the problem of the present inventioncan be solved. Therefore, in the present invention, it has been foundthat the action effects that is considered to be unpredictable by thoseskilled in the art are exerted in spite of employing the configurationcontrary to the conventional technical common sense as described above.Accordingly, it can be said that the present invention is not aninvention that could have been easily made by those skilled in the artwho had been in contact with the description of particularly JP2010-269985 A.

In addition, the addition amount of the silane coupling agent used inthe first reaction step is not particularly limited, but is, relative to100% by mass of the silica particles contained in the raw colloidalsilica, preferably 0.5 to 10% by mass, more preferably 1 to 5% by mass,and furthermore preferably 1 to 3% by mass. When the addition amount ofthe silane coupling agent is 0.5% by mass or more, the surfaces of thesilica particles can be sufficiently anionized, and an excellentperformance can be exerted in a case of being used as a polishing agent(abrasive grains in a polishing composition). On the other hand, whenthe addition amount of the silane coupling agent is 10% by mass or less,gelation with time of the reactant (modified colloidal silica) to beobtained can be prevented. Further, the amount of the organic solvent(hydrophilic solvent) used for dissolving the silane coupling agent is,relative to 100% by mass of the silane coupling agent, preferably around500 to 10000% by mass, and more preferably 1000 to 5000% by mass.

The temperature at which the silane coupling agent is added is notlimited, but is preferably in the range of room temperature (around 20°C.) to the boiling point of the reaction solvent. The reaction time isnot also limited, but is preferably 10 minutes to 10 hours, and morepreferably 30 minutes to 5 hours. However, from the viewpoint ofterminating the hydrolysis of the coupling agent, it is preferred thatthe first reaction step is performed under a condition that atemperature condition of 90° C. or more is continued for 30 minutes ormore. The pH at the time of addition is also not limited, but ispreferably 7 or more to 11 or less.

(Second Reaction Step)

In the second reaction step, the reactant (in which a silane couplingagent having a functional group chemically convertible to a sulfonicacid group is bonded to surfaces of silica particles), which has beenobtained in the above-described first reaction step, is treated. As aresult, the “functional group chemically convertible to a sulfonic acidgroup” possessed by the silane coupling agent is converted to a sulfonicacid group.

The specific form of the above-described “treatment” for converting the“functional group chemically convertible to a sulfonic acid group”possessed by the silane coupling agent to a sulfonic acid group is notparticularly limited, and can be appropriately selected depending on thestructure of the silane coupling agent to be used. For example, in thefirst reaction step, in a case where the above-described 1) a silanecoupling agent having a sulfonic acid ester group convertible to asulfonic acid group by hydrolysis is used, by subjecting the reactant toa hydrolysis treatment, the functional group (sulfonic acid ester group)possessed by the silane coupling agent can be hydrolyzed. As a result,the sulfonic acid ester group is converted to a sulfonic acid group.

Further, in the first reaction step, in a case where the above-described2) a silane coupling agent having a mercapto group and/or a sulfidegroup convertible to a sulfonic acid group by oxidation is used, bysubjecting the reactant to an oxidation treatment, the functional group(mercapto group and/or sulfide group) possessed by the silane couplingagent can be oxidized. As a result, the mercapto group or the sulfidegroup is converted to a sulfonic acid group.

In order to subject the reactant to an oxidation treatment, for example,the reactant may be reacted with an oxidizing agent. Examples of theoxidizing agent include nitric acid, hydrogen peroxide, oxygen, ozone,organic peracid (percarboxylic acid), bromine, hypochlorite, potassiumpermanganate, and chromic acid. Among these oxidizing agents, hydrogenperoxide and organic peracid (peracetic acid, and perbenzoic acids) arepreferred in the point of being relatively easy to handle and beingfavorable in the oxidation yield. Further, in consideration of thesubstances by-produced in the reaction, it is most preferred to usehydrogen peroxide. From the viewpoint of ensuring the amount requiredfor the reaction and decreasing the remaining oxidizing agent, theaddition amount of the oxidizing agent is preferably 3 to 5 mol timesthe amount of the silane coupling agent. By adjusting the additionamount of the oxidizing agent to a value within such a range, theresidual oxidizing agent concentration in the modified colloidal silicato be obtained can be minimized. The specific numerical value of theresidual oxidizing agent concentration in the modified colloidal silicato be obtained is not particularly limited, but is preferably 1000 ppmby mass or less, more preferably 700 ppm by mass or less, andparticularly preferably 500 ppm by mass or less. Herein, when theresidual oxidizing agent concentration in the modified colloidal silicato be obtained exceeds 1000 ppm by mass, in storing and transporting themodified colloidal silica itself, or the polishing composition in whichthe modified colloidal silica have been added as a polishing agent(abrasive grains), in a state of being enclosed in a sealed container,there may be a case where an oxidizing agent such as hydrogen peroxideis decomposed, gas such as oxygen is generated, and the internalpressure of the container increases. On the other hand, as describedabove, by decreasing the residual oxidizing agent concentration in themodified colloidal silica to be obtained, the risk of such an increaseof the internal pressure is reduced, therefore, this is preferred.Further, in a case of being used as a polishing composition, there isalso an advantage that the occurrence of a problem such as waferdishing, which can be generated when a large amount of oxidizing agentis contained, can be suppressed. In addition, as to the colloidal silicaand the silane coupling agent, each of them has a structure that isstable in the oxidation reaction except for the functional groups to beoxidized (converted) to sulfonic acid groups, therefore, by-products arenot present.

In a case where the modified colloidal silica obtained according to theabove-described method contains a solvent other than water, in order toimprove the long-term storage stability of the modified colloidalsilica, the dispersing medium mainly containing a reaction solvent maybe replaced with water as needed. In addition, this water replacementmay be performed after the addition of the silane coupling agent andbefore the addition of the oxidizing agent. The method for replacing thesolvent other than water with water is not particularly limited, and forexample, a method in which water is added dropwise by a fixed amountwhile heating the modified colloidal silica can be mentioned. Further, amethod in which the modified colloidal silica is separated from thesolvent other than water by precipitation and separation,centrifugation, or the like, and then redispersed in water can also bementioned.

(Cationically-Modified Colloidal Silica)

As described above, the present invention has been explained in detailby taking as an example the embodiment in which the modified colloidalsilica is anionically modified with a sulfonic acid group, but themodified colloidal silica according to the present invention may becationically modified (cationic modified colloidal silica). In order toobtain cationically-modified modified colloidal silica, in the similarmanner as in the above, to the raw colloidal silica in which the contentof microparticles is decreased, a silane coupling agent having an aminogroup or a quaternary cation group may be reacted under the similarconditions as those in the first reaction step.

At this time, examples of the silane coupling agent to be used includeN-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane,N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane,N-(β-aminoethyl)-γ-aminopropyltriethoxysilane,γ-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane,γ-triethoxysilyl-N-(α,γ-dimethyl-butylidene)propylamine,N-phenyl-γ-aminopropyltrimethoxysilane, hydrochloride ofN-(vinylbenzyl)-β-aminoethyl-γ-aminopropyltriethoxysilane and octadecyldimethyl-(γ-trimethoxysilylpropyl)-ammonium chloride. Among them, sincethe reactivity with the colloidal silica is favorable,N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane,N-(β-aminoethyl)-γ-aminopropyltriethoxysilane,γ-aminopropyltriethoxysilane, and γ-aminopropyltrimethoxysilane arepreferably used. Further, in the present invention, the silane couplingagent may be used singly alone or may be used in combination of two ormore kinds thereof.

The modified colloidal silica according to the present invention has asmall content of the microparticles including the silica particles ofwhich the surfaces are modified (anionically modified or cationicallymodified). When expressing this quantitatively, according to the presentinvention, the following two forms are further provided depending on thetype of the modification treatment.

-   -   Modified colloidal silica being anionically modified, in which        when a SiN wafer is subjected to an immersion treatment under a        pH 2 condition and then washed with pure water, the number of        the particles having a particle size of less than 40% of a        volume average particle size adhering to a surface of the SiN        wafer is 50% or less relative to the number of the particles        having a particle size of 40% or more of a volume average        particle size adhering similarly; and    -   Modified colloidal silica being cationically modified, in which        when an aluminosilicate glass wafer is subjected to an immersion        treatment under a pH 4 condition and then washed with pure        water, the number of the particles having a particle size of        less than 40% of a volume average particle size adhering to a        surface of the aluminosilicate glass wafer is 50% or less        relative to the number of the particles having a particle size        of 40% or more of a volume average particle size adhering        similarly.

The above-described proportion is preferably 30% or less, morepreferably 10% or less, and furthermore preferably 5% or less. On theother hand, the lower limit value of this proportion is not particularlylimited, but is, for example, 0.1% or more. Further, the measurementmethod of this proportion is as follows (regarding the anionicallymodified colloidal silica, the measurement method is described as“condition 8” also in a section of Examples).

(Adhesion Observation Test of SiN Wafer for Anionic Modified ColloidalSilica)

Apparatus: Scanning electron microscope SU 8000 (manufactured by HitachiHigh-Technologies Corporation)

Procedures: The obtained anionic modified colloidal silica is diluted tohave a silica concentration of 14% by mass and adjusted to pH 2 with apH adjusting agent. A SiN wafer is immersed for 10 seconds, and thenshaken in pure water for 30 seconds. Subsequently, the SiN wafer isdried thoroughly with N₂ gas, and then ten viewing fields are observedat a magnification of 100000 times using a scanning electron microscopeSU 8000.

(Adhesion Observation Test of Aluminosilicate Glass Wafer for CationicModified Colloidal Silica)

Apparatus: Scanning electron microscope SU 8000 (manufactured by HitachiHigh-Technologies Corporation)

Procedures: The obtained cationic modified colloidal silica is dilutedto have a silica concentration of 1% by mass and adjusted to pH 4 with apH adjusting agent. An aluminosilicate glass wafer is immersed for 10seconds, and then shaken in pure water for 30 seconds. Subsequently, thealuminosilicate glass wafer is dried thoroughly with N₂ gas, and thenten viewing fields are observed at a magnification of 100000 times usinga scanning electron microscope SU 8000.

In addition, in the above-described adhesion observation test, since theinterval between adhered particles differs depending on the particlesize of silica particles, when the test is performed, the silicaconcentration in the colloidal silica can be arbitrarily changed so thatthe observation is easily performed, and this change has no effect onthe measurement results.

Further, the modified colloidal silica obtained by the production methodof the present invention is preferred also in the point that the contentof metal impurities is reduced. Herein, examples of the metal impuritiesinclude an alkali metal such as sodium, and potassium; an alkaline earthmetal such as calcium, and magnesium; and a heavy metal or light metalsuch as aluminum, iron, titanium, nickel, chromium, copper, zinc, lead,silver, manganese, and cobalt. In the modified colloidal silicaaccording to the preferred embodiment of the present invention, thetotal content of the metal impurities is 1 ppm by mass or less. Thetotal content is preferably 0.5 ppm by mass or less. In addition, themethod for measuring the total content of the metal impurities isperformed in accordance with the description in Examples describedlater. Further, the modified colloidal silica is preferred because ofcontaining no corrosive halogen element such as chlorine, and bromine.

The particle size of the silica particles contained in the modifiedcolloidal silica according to the present invention is not particularlylimited, and is for example, 1000 nm or less, preferably 5 to 500 nm,and more preferably 10 to 300 nm. In addition, the particle size of thesilica particles means the volume average particle size based on Heywooddiameter (equivalent circle diameter) as measured by the techniquedescribed in Examples described later.

The modified colloidal silica according to the present invention isexcellent in the long-term dispersion stability in a wide pH range. Thestability of the silica sol can be evaluated by measuring the zetapotential of the silica sol. The zeta potential is a potentialdifference generated at the interface between solid and liquid, whichare in contact with each other, when the solid and the liquid are movedrelatively to each other. As the absolute value of the zeta potentialincreases, the repulsion between the particles becomes stronger and thestability of the particles becomes higher, and as the absolute value ofthe zeta potential approaches zero, the particles tend to aggregate moreeasily.

In particular, among the modified colloidal silica according to thepresent invention, the anionically-modified modified colloidal silicahas high stability in the acidic region. Since a coupling agent havingan anionic group is used as a modifier when the anionically-modifiedmodified colloidal silica is obtained, the zeta potential is negativepotential (−15 mV or less) when the dispersing medium is under an acidiccondition of pH 2 or more, and even if the dispersing medium is under anacidic condition, the dispersion stability is high. As described above,since the absolute value of the zeta potential is large, the dispersionstability is high, and along with this, the kinematic viscosity of themodified colloidal silica is also small.

The modified colloidal silica according to the present invention can beused for various applications, such as a polishing agent (abrasivegrains) contained in a polishing composition, and a paper coating agent,and can be stably dispersed for a long time in a wide pH range. Asdescribed above, when the modified colloidal silica according to thepresent invention, in which the proportion of the microparticlesadhering to a wafer is kept low, is used particularly as a polishingagent (abrasive grains) for CMP polishing of a semiconductor wafer, thefluctuation with time of the polishing speed can be minimized (excellentin the stability with time), therefore, the modified colloidal silica isextremely suitably used for the applications, and can sufficiently copealso with the high performance requirements accompanying theminiaturization.

EXAMPLES

The present invention will be described in more detail using thefollowing Examples and Comparative Examples. However, the technicalscope of the present invention is not limited only to the followingExamples.

Example 1

In a flask, 4080 g of methanol, 610 g of water, and 168 g of 29% by massammonia aqueous solution were mixed, the temperature of the resultantmixture was kept at 20° C., and into the mixture, a mixture of 135 g ofmethanol and 508 g of tetramethoxysilane (TMOS) was added dropwise over25 minutes. After that, the resultant mixture was subjected to heatconcentration and water replacement under a condition of pH 7 or more,and 1000 g of 19.5% by mass silica sol was obtained. It was confirmed bygas chromatography (the following condition 1) that the methanolconcentration at that time was less than 1% by mass (lower than thedetection limit).

(Condition 1: Measurement Conditions for Methanol Concentration UsingGas Chromatography)

Apparatus: Gas chromatography GC-14B (manufactured by ShimadzuCorporation)

Measurement: 4 μL of a sample is taken out using a 10 μL syringe andinjected to the apparatus. The methanol concentration is calculated fromthe amount of moisture and the amount of methanol, which are obtained inthe measurement.

On the other hand, the silica sol obtained by the above procedure wasobserved with a scanning electron microscope (SEM) (the followingcondition 2) (FIG. 1 ), and when the particle size distribution wasanalyzed by using image analysis software (the following condition 3)based on the SEM picture, the number distribution ratio of themicroparticles having a size of 40% or less of the volume averageparticle size by SEM image analysis was less than 1%. In addition, whenthe surface state of the silica particles was observed with atransmission electron microscope (TEM) (the following condition 4), thesurface of the silica particles showed a smooth state (FIG. 2 ).

(Condition 2: Conditions for SEM Observation)

Apparatus: Scanning electron microscope 54700 (manufactured by HitachiHigh-Technologies Corporation)

Procedures: The sample, which is obtained by dispersing silica sol in anorganic solvent and subjecting the resultant mixture to dry-solidifyingon a sample table, is placed on the main body of the apparatus, andirradiated with an electron beam at 12 kV by the apparatus, and then tenviewing fields are observed at a magnification of 100000 times.

(Condition 3: Conditions for Image Analysis Based on SEM picture)

Apparatus: Image analysis software MacView Ver. 4 (manufactured byMountech Co., Ltd.)

Procedures: Using the taken SEM picture, 500 particles are counted bythe apparatus. After that, the volume average particle size based onHeywood diameter (equivalent circle diameter) is calculated, and theparticle size distribution in terms of the proportion of the number ofparticles is calculated.

(Condition 4: Conditions for TEM Observation)

Apparatus: Transmission electron microscope HD-2700 (manufactured byHitachi High-Technologies Corporation)

Procedures: Silica sol is dispersed in an organic solvent, the resultantmixture is added dropwise on a surface of dedicated Cu mesh anddry-solidified, the dry-solidified sample is irradiated with an electronbeam at 200 kV by the apparatus, and then ten viewing fields areobserved at a magnification of 400000 times.

Subsequently, into 1000 g of the silica sol obtained in the aboveprocedure, 1.7 g of 29% by mass ammonia water was added to lower theviscosity, and into the resultant mixture, 2.5 g of 3-mercaptopropyltrimethoxysilane (product name: KBM-803, manufactured by Shin-EtsuChemical Co., Ltd.) separately mixed with 22.5 g of methanol was addeddropwise at a flow rate of 5 mL/min, and then the resultant mixture washeated, after the boiling, water replacement was performed for 6 hours.It was confirmed by the similar technique as in the above (gaschromatography method) that the methanol concentration at that time wasa value outside the detection limit.

Next, the reaction mixture was once cooled down to 25° C., and then intothe cooled mixture, 4.2 g of 31% by mass hydrogen peroxide water wasadded, and the resultant mixture was boiled again. After the boiling,water replacement was performed for 4 hours, and then the resultantmixture was cooled down to room temperature, and the sulfonic acid(anionic) modified colloidal silica of the present Example was obtained.

The amount of the impurities of 13 metal elements in the modifiedcolloidal silica thus obtained was measured by the metal impurityconcentration measurement (the following condition 5) using aninductively coupled plasma (ICP) emission spectrometer, and in addition,the amount of the supernatant Si in the obtained modified colloidalsilica was measured by the supernatant Si concentration measurement (thefollowing condition 6) using an inductively coupled plasma (ICP)emission spectrometer. Note that the supernatant Si concentration is avalue obtained by measuring the supernatant, which has been obtained bycentrifuging the modified colloidal silica, using an inductively coupledplasma (ICP) emission spectrometer, and the fact that this value variesover time means that aggregation of microparticles and incorporation ofmicroparticles to large particles are generated, and the physicalproperties are changed.

Further, a test of the SiN polishing speed was performed with a 300 mmCMP one-side polishing device (manufactured by Ebara Corporation) (thefollowing condition 7).

In addition, an adhesion test on the SiN wafer was also performed, andthe adhered wafer was observed by using a scanning electron microscope(manufactured by Hitachi High-Technologies Corporation) (the followingcondition 8). Further, the proportion of the number of small particleswas analyzed based on the SEM picture (the above condition 3).

In addition, by the particle size distribution analysis based on the SEMpicture using a scanning electron microscope (SEM), and by the surfaceshape observation at high magnification using a transmission electronmicroscope (TEM), the verification of the physical properties of thefinished product was performed. From the results of the surface shapeobservation with TEM, changes in the surface characteristics of thesilica particles were not observed even after performing the first andthe second reaction steps.

(Condition 5: Conditions for Metal Impurity Concentration MeasurementUsing ICP Emission Spectrometer) Measuring Apparatus:

Ni and Cu: Agilent 7500cs ICP-MS (manufactured by Agilent Technologies,Inc.)

Metals other Ni and Cu: ICPS-8100 (manufactured by Shimadzu Corporation)

Procedures: 10 ml of a sample is collected, into the collected sample, 3ml of nitric acid and 10 ml of hydrofluoric acid are added, and theresultant mixture is evaporated and dry-solidified. After thedry-solidifying, 0.5 ml of nitric acid and around 20 ml of ultrapurewater are added, and the resultant mixture is heated until steam isgenerated. The whole amount is collected, adjusted to 50 g withultrapure water, and measurements are performed by using each of theabove apparatuses.

(Condition 6: Conditions for Supernatant Si Concentration MeasurementUsing ICP Emission Spectrometer)

Centrifugal device: High-performance high-speed refrigerated centrifuge,Avanti HP-30I (manufactured by Beckman Coulter, Inc.)

ICP measuring apparatus: ICP-AES SPS3510 (manufactured by HitachiHigh-Tech Science Corporation)

Procedures: Modified colloidal silica is placed in a dedicated resintube, and centrifugation is performed at 26000 rpm for 2 hours.Subsequently, a calibration curve is drawn with Si standard samples of0, 25, 50, and 75 ppm by ICP-AES, 1 g of the supernatant after thecentrifugation is collected, and diluted with ultrapure water by 20times, and then measurement is performed by the apparatus.

(Condition 7: Test Conditions for SiN Polishing Speed by 300 mm CMPOne-Side Polishing Device)

Apparatus: 300 mm CMP one-side polishing device (manufactured by EbaraCorporation)

Polishing pad: polyurethane foam

Polishing wafer: 300 mm SiN bare wafer

Rotation speed: 60 rpm

Pressure: 70 hPa

Slurry flow rate: 300 mL/min

Polishing time: 60 sec

Polishing speed [A/min]=Amount of the change in film thickness whenpolished for 1 minute

The polishing speed was calculated by dividing each wafer thicknessdifference by the polishing time, the each wafer thickness differencewas obtained by measuring before and after the polishing by using anoptical interference-type film thickness measuring device.

(Condition 8: Adhesion Observation Test of SiN Wafer)

Apparatus: Scanning electron microscope SU 8000 (manufactured by HitachiHigh-Technologies Corporation)

Procedures: The obtained modified colloidal silica is diluted to have asilica concentration of 14% by mass and adjusted to pH 2 with a pHadjusting agent. A SiN wafer is immersed for 10 seconds, and then shakenin pure water for 30 seconds. Subsequently, the SiN wafer is driedthoroughly with N₂ gas, and then ten viewing fields are observed at amagnification of 100000 times using a scanning electron microscope SU8000.

Comparative Example 1: Corresponding to Example 1 of JP 2010-269985 A

Into a mixture of 551.5 g of pure water, 550.2 g of 26% by mass ammoniawater, and 9047 g of methanol in a flask, a mixture of 1065.5 g oftetramethoxysilane (TMOS), and 289.1 g of methanol was added dropwiseover 55 minutes while maintaining the mixture temperature at 35° C., asa result of which silica sol containing water and methanol as adispersing medium was obtained.

The silica sol obtained in the above procedure was heated andconcentrated to 3500 mL under normal pressure. When the methanolconcentration of the mixture was measured in the similar manner as inthe above, 71% by mass was obtained. Further, the silica sol obtained inthe above procedure was observed using a scanning electron microscope(SEM) in the similar manner as in the above (FIG. 3 ), and when theparticle size distribution was analyzed by using image analysis softwarebased on the SEM picture, the number distribution ratio of themicroparticles having a size of 40% or less of the volume averageparticle size by SEM image analysis was 47.6%. In addition, when thesurface state of the silica particles was observed with a transmissionelectron microscope (TEM) in the similar manner as in the above,existence of an uneven state was confirmed on the surfaces of the silicaparticles (FIG. 4 ).

Subsequently, into 3500 mL of the silica sol obtained in the above, 24.2g of 3-mercaptopropyl trimethoxysilane (product name: KBM-803,manufactured by Shin-Etsu Chemical Co., Ltd.) was added, and theresultant mixture was refluxed at a boiling point and heat-aged. Afterthat, methanol and ammonia were replaced with water while adding purewater in order to keep the capacity constant, and when the pH became 8or less, the temperature of the silica sol was once lowered to roomtemperature. Next, into the resultant silica sol, 37.5 g of 35% by masshydrogen peroxide water was added, and the resultant mixture was heatedagain, the reaction was continued for 8 hours, and the resultant mixturewas cooled down to room temperature, subsequently modified colloidalsilica of the present Comparative Example was obtained.

As to the modified colloidal silica thus obtained, in the similar manneras in the above, the amount of the impurities of 13 metal elements wasmeasured by the metal impurity concentration measurement using aninductively coupled plasma (ICP) emission spectrometer, and in addition,the amount of the supernatant Si in the obtained modified colloidalsilica was measured by the supernatant Si concentration measurementusing an inductively coupled plasma (ICP) emission spectrometer.

Further, in the similar manner as in the above, a test of the SiNpolishing speed was performed using a 300 mm CMP one-side polishingdevice (manufactured by Ebara Corporation).

Subsequently, an adhesion test on the SiN wafer was also performed, andthe adhered wafer was observed by using a scanning electron microscope(manufactured by Hitachi High-Technologies Corporation). Further, theproportion of the number of small particles based on the SEM picture wasanalyzed.

In addition, in the similar manner as in the above, by the particle sizedistribution analysis from the SEM picture with a scanning electronmicroscope (SEM), and by the surface observation using a transmissionelectron microscope (TEM), the verification of the physical propertiesof the finished product was performed. From the results of the surfaceobservation with TEM, changes in the surface characteristics of thesilica particles were not observed even after performing the first andthe second reaction steps.

Comparative Example 2

Into a mixture of 133 g of pure water, 64.8 g of 29% by mass ammoniawater, and 1223 g of methanol, a mixture of 1015 g of tetramethoxysilane(TMOS) and 76 g of methanol, and 239 g of pure water were simultaneouslyadded dropwise over 150 minutes while maintaining the mixturetemperature at 35° C., as a result of which silica sol containing waterand methanol as a dispersing medium was obtained. When the methanolconcentration of the mixture was measured in the similar manner as inthe above procedure, 65% by mass was obtained. Further, the silica solobtained in the above procedure was observed using a scanning electronmicroscope (SEM) (FIG. 5 ), and when the particle size distribution wasanalyzed by using image analysis software based on the SEM picture, thenumber distribution ratio of the microparticles having a size of 40% orless of the volume average particle size by SEM image analysis was83.9%. In addition, when the surface state of the silica particles wasobserved with a transmission electron microscope (TEM) in the similarmanner as in the above procedure, existence of an uneven state wasconfirmed on the surfaces of the silica particles (FIG. 6 ).

Subsequently, into the silica sol obtained in the above procedure(around 2000 g in terms of 19.5% by mass), 5.0 g of 3-mercaptopropyltrimethoxysilane (product name: KBM-803, manufactured by Shin-EtsuChemical Co., Ltd.) mixed with 45.0 g of methanol was added dropwise ata flow rate of 5 mL/min, and then the resultant mixture was heated,methanol and ammonia were replaced with water while adding pure water inorder to keep the capacity constant, and when the pH became 8 or less,the temperature of the silica sol was once lowered to room temperature.After the resultant silica sol was cooled down to room temperature, intothe cooled silica sol, 8.4 g of 31% by mass hydrogen peroxide water wasadded, and the resultant mixture was boiled again. After the boiling,water replacement was performed for 4 hours, and then the resultantmixture was cooled down to room temperature, as a result of whichmodified colloidal silica of the present Comparative Example wasobtained.

As to the modified colloidal silica thus obtained, in the similar manneras in the above procedure, the amount of the impurities of 13 metalelements was measured by the metal impurity concentration measurementusing an inductively coupled plasma (ICP) emission spectrometer, and inaddition, the amount of the supernatant Si in the obtained modifiedcolloidal silica was measured by the supernatant Si concentrationmeasurement using an inductively coupled plasma (ICP) emissionspectrometer.

Further, in the similar manner as in the above procedure, a test of theSiN polishing speed was performed using a 300 mm CMP one-side polishingdevice (manufactured by Ebara Corporation).

Subsequently, an adhesion test on the SiN wafer was also performed, andthe adhered wafer was observed by using a scanning electron microscope(manufactured by Hitachi High-Technologies Corporation). Further, theproportion of the number of small particles based on the SEM picture wasanalyzed.

In addition, in the similar manner as in the above procedure, by theparticle size distribution analysis based on the SEM picture using ascanning electron microscope (SEM), and by the surface observation usinga transmission electron microscope (TEM), the verification of thephysical properties of the finished product was performed. From theresults of the surface observation with TEM, changes in the surfacecharacteristics of the silica particles were not observed even afterperforming the first and the second reaction steps.

TABLE 1 Physical properties of Physical properties of silica silicaparticles after After high temperature particles before addition ofcompletion of modified Immediately after acceleration test at couplingagent colloidal silica production 80° C. for 1 week number NumberSurface ratio of SiN Physical percentage of Methanol distribution shapeof SiN polishing properties/ microparticles concentration in SiN wafersilica Supernatant polishing Supernatant speed rate [%] [% by mass]adhesion [%] particle Si [ppm] rates [%] Si [ppm] ratio [%] changeExample 1 0.17 <1 (Not 0.15 Smooth 200 100 200 100 No detected)(Reference value) Comparative 47.6 71 65.3 Uneven 700 104 400 92 YesExample 1 Comparative 83.9 65 88.2 Uneven 2000  92 1700 88 Yes Example 2

From the results shown in Table 1, the modified colloidal silica ofExample 1 produced by the production method according to the presentinvention showed smooth surface shape of silica particles, and theamount of the microparticles adhering to the SiN wafer was largelyreduced. In addition, the amount of the supernatant Si (depending on theamount of microparticles) contained in the obtained modified colloidalsilica was also reduced, and further, the amount of supernatant Si didnot change also over time. As a result, it was confirmed that the ratioof the SiN polishing rates also does not change over time, and themodified colloidal silica is excellent extremely in the stability withtime.

On the other hand, the modified colloidal silica produced by theproduction methods of Comparative Examples 1 and 2 had unevenness on thesurfaces of the silica particles, and the amount of the microparticlesadhered to the SiN wafer was also large. In addition, the amount of thesupernatant Si contained in the obtained modified colloidal silica wasalso large, and further, the amount of supernatant Si changed over time.As a result, it was confirmed that the ratio of the SiN polishing ratesalso fluctuated largely over time, and this resulted in being inferiorin the stability with time.

Further, the measurement results of the above-described metal impurityamount are shown in the following Table 2.

TABLE 2 Unit: ppb by Comparative Comparative mass Example 1 Example 1Example 2 Ni 0.1 0.1 0.2 Cu <0.05 <0.05 <0.05 Zn <5 <5 <5 Ag <3 <3 <3 Al<5 <5 <5 Ca 2.1 1.9 1.7 Cr <5 <5 <5 Fe <10 <10 <10 K <55 <55 <55 Mg <3<3 <3 Na 12.6 13.2 12.4 Pb <14 <14 <14 Ti <5 <5 <5

From the results shown in Table 2, it can be understood that themodified colloidal silica according to the present invention has anextremely small content of metal impurities.

This application is based on Japanese Patent Application No.2015-008049, filed with the Japan Patent Office on Jan. 19, 2015, theentire content of which is hereby incorporated by reference.

The invention claimed is:
 1. A method for producing modified colloidalsilica, comprising the steps of: distilling off an organic solventcoexisting with colloidal silica in colloidal silica having an organicsolvent concentration of 1% by mass or more under a condition of pH 7 ormore so that a residual organic solvent concentration is less than 1% bymass to obtain raw colloidal silica; lowering a viscosity of the rawcolloidal silica by adding an alkaline solution to the raw colloidalsilica; and modifying the raw colloidal silica to obtain modifiedcolloidal silica; wherein the modifying comprises: a first reaction stepof heating the raw colloidal silica in the presence of a silane couplingagent having a functional group chemically convertible to a sulfonicacid group to obtain a reactant, wherein a mixture of the silanecoupling agent and a hydrophilic solvent is added to the raw colloidalsilica; and a second reaction step of treating the reactant to convertthe functional group to a sulfonic acid group.
 2. The method forproducing modified colloidal silica according to claim 1, wherein thefunctional group is a mercapto group.
 3. A method for producing modifiedcolloidal silica according to claim 1, wherein the treatment is anoxidation treatment using an oxidizing agent.
 4. The method forproducing modified colloidal silica according to claim 3, wherein anaddition amount of the oxidizing agent in the oxidation treatment is 3to 5 mol times relative to an addition amount of the silane couplingagent, and a residual oxidizing agent concentration in the obtainedmodified colloidal silica is 1000 ppm by mass or less.
 5. The method forproducing modified colloidal silica according to claim 3, wherein theoxidizing agent is hydrogen peroxide.
 6. The method for producingmodified colloidal silica according to claim 1, wherein the firstreaction step is performed under a condition that a temperaturecondition of 90° C. or more is continued for 30 minutes or more.
 7. Themethod for producing modified colloidal silica according to claim 1,wherein the alkaline solution is ammonia water.